1. Field of the Invention
This invention relates generally to fuel injection systems for internal combustion engines, and in particular to self-tuning fuel injection systems such as systems designed for aftermarket and high performance use.
2. Background Art
Fuel injection systems precisely meter fuel, thereby allowing optimal fuel-air mixture to be consistently delivered across the full spectrum of driving conditions. Fuel injection provides increased horsepower, higher torque, improved fuel economy, quicker cold starting, and other benefits as compared to older carburetion fuel delivery systems. Fuel injection systems use one or more fuel injectors, which are electromechanical devices that meter and atomize fuel. In each injector, application of an electrical current to a coil lifts a spring-loaded needle within a pintle valve off its seat, thereby allowing fuel under pressure to be sprayed through an injector nozzle to form a cone pattern of atomized fuel.
Electronic control is the most common manner for governing the rate of fuel injection. A microprocessor- or microcontroller-based computer system is included within an engine control unit (ECU). The computer controls fuel delivery by rapidly cycling on and off fuel injectors. The computer generates periodic pulse signals for each of the injectors, with “on” pulses for firing the fuel injectors. The duration of the “on” pulses determines fuel flow rate.
Fuel injector pulsing is controlled primarily as a function of engine speed, engine load, exhaust oxygen levels, and sometimes manifold air temperature (for air density compensation), coolant temperature (i.e., for simulating carburetor choke function) or throttle position (i.e., for simulating carburetor accelerator pump circuit operation). One or more driver circuits, which may be located within the ECU, amplify and condition the pulse signals to be suitable for use with the fuel injectors. The cycle wavelength is a function of engine speed, and the pulse widths of the “on” pulses are a function of engine load. Engine speed is typically determined by a distributor output, a tachometer output, or a crankshaft sensor. Engine load is typically determined with either a mass airflow sensor or a manifold absolute pressure (MAP) sensor.
Based on the engine speed and load input signals, the computer generates the fuel injector pulse signals. The fuel injector pulse signals are initially based on target air-fuel ratio values, which are compensated for the volumetric efficiency of the engine at its operating speed and load. Target air-fuel ratios and volumetric efficiency coefficients may be stored in one or more look-up tables in volatile or non-volatile computer memory and are accessed using engine load and speed as input indices. The use of look-up tables allows for rapid response by the ECU to various vehicle operating conditions without the need for extensive time-consuming calculations. Controlling the fuel injection directly from the look-up tables is referred to as open-loop control.
However, when the ECU operates in a closed-loop control mode, the actual fuel injector pulse signals may vary from those derived directly from the look-up tables based on actual engine operating conditions. In closed-loop control, the amount of oxygen present in the exhaust gas is measured, which provides an indication of whether the engine is running too rich, too lean, or stoichiometrically. The fuel rate supplied to the engine is corrected by the ECU based on the input from an oxygen sensor in an attempt to equate the actual air-fuel ratio to the stored target air-fuel ratio. Such closed-loop correction is sometimes referred to as short-term fuel trim, as the corrections are momentary in nature and are not stored.
In some ECU systems, one or more look-up tables may occasionally be updated based on the short-term fuel trim derived during closed-loop control. Such correction of the look-up tables is also referred to as long-term fuel trim. Because long-term corrections are made to the look-up tables stored in non-volatile memory, the duration of fuel injection is affected in both open-loop and closed-loop control modes for better overall fuel control.
Although short-term fuel trim is relatively responsive to rapid changes detected by the oxygen sensor, closed-loop control still involves an inherent feedback lag time. Additionally, although closed-loop control is ideal for cruising, idling, and light acceleration conditions, it is not suitable for use under all operating conditions. For example, only open-loop control is appropriate for use during wide-open throttle conditions, during hard acceleration, when starting the engine, or when the engine is cold. For these reasons, long-term fuel trim auto-tuning, which improves both open-loop and closed-loop operation, is desirable.
3. Identification of Objects of the Invention
A primary object of the invention is to provide a method and an electronic fuel injection control system that provides superior performance by intelligently applying long-term fuel trim corrections that minimize discontinuities in the look-up tables.
Another object of the invention is to provide a method and an electronic fuel injection control system that provides superior performance by correcting discontinuities in the look-up tables independently of long-term fuel trim corrections.